First time I've felt lost in a while: AA gets check-3bet

[Deamon2]

I've been feeling a little animosity from sb because I've tried to steal his limped sb from him a couple of times. We have fought HU, but we have been in multi-way pots with each other in between them where he played normally. Everyone else in the hand is usual fair for a good game.

Party Poker 0.5/1 Hold'em (6 max, 6 handed) converter

Preflop: Hero is BB with A[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/spade.gif[/img].
<font color="#666666">2 folds</font>, CO calls, Button calls, SB completes, <font color="#CC3333">Hero raises</font>, CO calls, Button calls, SB calls.

Flop: (8 SB) K[img]/images/graemlins/spade.gif[/img], 4[img]/images/graemlins/diamond.gif[/img], 5[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
SB checks, <font color="#CC3333">Hero bets</font>, CO folds, <font color="#CC3333">Button raises</font>, <font color="#CC3333">SB 3-bets</font>, Hero folds, Button calls.

I think sb could do this with a king, 2 pair, or maybe a set. I'd doubt a set because most players like to trap with them, but it's possible he noticed that the table is calling down with pretty much everything. When I was in the hand, I freaked out and folded, but looking back it doesn't seem as clear as it did when I was playing. I definitely can't call here, but capping feels so wrong. I doubt this play would be any major leak (it doesn't occur often) but I'm curious as to what you people think about it.